1. Field of the Invention
The present invention relates to an image processing method for processing the color space of image data according to the color reproducing region of the image output apparatus, and an apparatus adapted for executing such image processing method.
2. Related Background Art
With the extensive spreading of personal computers and the similar equipment in recent years, there are increasing the opportunities of preparing or processing a color image on the computer, and observing the image on the color display or releasing the prepared image on paper by the hard copying apparatus or ink jet printer.
In such case, the color reproducing ability varies depending on the image output device (for example hard copying apparatus or ink jet printer), and there is therefore no guarantee that the color observed on the monitor is faithfully reproduced on the hard copy. In particular, as the color reproduction range of the hard copying device is generally narrower than that of the color monitor, the color which can be represented on the monitor but is outside the color reproduction range of the hard copying device cannot, in most cases, be reproduced on the hard copy.
For avoiding such drawback, there can be conceived a method, for example, of preparing a look-up table by predetermining, for all the combinations of color signals of the color image, whether they belong to the color reproduction range of the image output device and providing an alarm in case the entered color signal is outside the color reproduction range.
Such method is however associated with a drawback that a very large memory capacity is required for such look-up table because the number of combinations of the color signals is enormous. The required memory capacity becomes even larger in case plural image output devices are employed, as the look-up table has to be prepared for each of such image output devices.
Also as already mentioned above, the color reproducing ability of the color hard copy varies depending on the system used. For example, while the silver halide photography can reproduce the image to a maximum density of about 2.0, the color copying machine can only reproduce the image to a maximum density of about 1.8.
Consequently, when a silver halide-based photograph is copied on a color copying machine, the high density areas of the photograph become saturated, so that the information originally contained in the image becomes eventually lost partially.
Stated differently, when an original image, of which information overflows the color reproduction range of the image output device, is copied with such device, the information originally contained in the image is inevitably lost.
For this reason, there have been conceived methods of color space compression, for converting the color signals of the input original image so as to be inside the color reproduction range of the image output device, in the course of supply of the input original image to the image output device.
However, as the color reproduction range of the image output device is predetermined in the beginning, such methods are incapable of adapting to the variation in the color reproduction range resulting from the machine-to-machine fluctuation, the difference in ambient conditions or the long-term performance variation of the machine.
In the conventional example explained above, a same value is used for different apparatus, though the color reproduction range is in fact different from apparatus to apparatus, because of the fluctuation of individual apparatus, difference in ambient condition of the site of installation and time-dependent variation of the performance.
Consequently, if the initially set color reproduction range is narrower than the actual range, the obtained image is represented in a range narrower than the available range, so that the actually available color reproduction range is not effectively utilized.
On the other hand, if the initially set color reproduction range is wider than the actual range, the total rendition may become saturated as the apparatus tries to reproduce the rendition which cannot be reproduced.
In such case, the color space compression explained above cannot be effectively utilized.